1. Field of the Invention
This invention relates to water absorbent compositions. This invention also relates to a process for preparing such compositions.
2. Discussion of the Related Art
Superabsorbents are hydrogels which are capable of absorbing many times their weight in water. Superabsorbent polymers, synthetic, part synthetic or hydrocolloid based, have been well known for over 30 years. Examples of synthetic superabsorbents are the crosslinked polyacrylamides and polyacrylic acids. Starch/polyacrylonitrile graft copolymers are part synthetic, while crosslinked carboxymethylcellulose and crosslinked carboxymethyl starches are examples of hydrocolloid superabsorbent polymers.
The use of superabsorbents has been contemplated in applications in which immobilization of water or aqueous fluids within a swollen, hydrogel network is required. For example, these polymers have been proposed as potentially useful in such products as diapers, sanitary napkins and the like, as well as protective coatings for seeds, controlled delivery vehicles for drugs, herbicides, pesticides and insulating materials. An extensive list of synthetic superabsorbents is given in U.S. Pat. No. 4,983,390, the entire contents of which are incorporated by reference herein.
In many regions of the world, and in particular in the more arid regions, there is a major need to conserve precious water resources and to maximize water's effectiveness in horticultural uses. Because of superabsorbents' high water absorption capacity and their ability to dry out and rehydrate many times in succession, they have been considered as potentially useful tools in the area of water conservation. However, synthetic and part synthetic superabsorbents suffer from the disadvantage that they are not biodegradable and sometimes contain residual and undesirable quantities of unreacted monomer. In addition, it has been reported that polyacrylamides, by themselves, do not reduce the rate at which water is lost by evaporation from soil. Letey. J. et al., Calif. Agric., May/June 1992, pp. 9-10; Tayel, M. Y. and El-Hady, O. A., Acta. Horticultrae, 119 (1981) pp. 247-256.
In contrast to synthetic and semi-synthetic superabsorbents, hydrocolloid based superabsorbents are readily degraded by bacteria, ultimately to carbon dioxide, water and other small molecules. For this reason, hydrocolloid based superabsorbents, such as the crosslinked carboxymethylated starch derivatives described in British Patent No. 1,576,475, are particularly attractive as candidates for helping conserve water resources.
However, the ability of a material to absorb large amounts of water does not necessarily guarantee that the material will be effective in water conservation uses. This is demonstrated by the example of polyacrylamides, which are superabsorbents, but which do not reduce the rate at which water is lost by evaporation from soil. Thus, polyacrylamides allow water to be added to soil less frequently, but not in lower amounts. The application of superabsorbents in water conservation depends upon their ability to reduce the rate at which water is lost from soil by evaporation.
Colligative properties, such as boiling point elevation and freezing point depression, depend upon the number of solute molecules in solution. Consequently, high molecular weight solutes are much less effective than low molecular weight solutes in influencing these properties. For example, 10 grams of a high molecular weight hydrocolloid is much less effective in elevating the boiling point of water than are 10 grams of sucrose because there are more molecules of sucrose in 10 grams of sucrose than there are molecules of hydrocolloid in 10 grams of hydrocolloid.
A superabsorbent composition having a reduced rate of water evaporation would be desirable. Such a composition could be added to soil to enhance water conservation.
Alginates are obtained from brown seaweed. They have been used in industry as food hydrocolloids for a long time. Structurally, alginates are linear polymers composed of mannuronic acid and guluronic acid. Functionally, alginates may be used as viscosifying or gelling agents in water-based systems. At low calcium ion concentrations, alginates function as viscosifying agents. At high calcium ion concentrations, alginates function as gelling agents.
Use of alginate as a thickener is relatively straightforward, provided that the polymer is properly hydrated and dispersed. Gel formation, normally induced at room temperature, may be brought about by controlled release of calcium ions into the system. A number of ways for obtaining this controlled release are well known by those skilled in the art. Many of these involve the use of sequestrants, typically sodium citrate, to compete with the alginate for the calcium as it is being released.
Alginates can also form gels and precipitates with acid. Propylene glycol alginates have enhanced acid stability. These alginates are often used as stabilizers in beverages, syrups, salad dressings and beer.
A by-product of manufacturing processes for alginate is alginate product which does not meet the specification requirements of the end use application. It would be highly desirable to provide a commercial use for such unusable alginate while simultaneously providing water absorbent products which reduce the rate of water evaporation from soil.